Sodium nitrite oxidation of hydrogen sulfide

ABSTRACT

A method for treating hydrogen sulfide in a solution includes providing the solution containing hydrogen sulfide. The method also includes adding sodium nitrite to the solution in an amount suitable to react with the hydrogen sulfide and treat the hydrogen sulfide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/623,112 filed Jun. 14 2017, which is a continuation of U.S.application Ser. No. 14/197,034 filed Mar. 4, 2014, which is acontinuation of U.S. Pat. No. 8,702,994 issued Apr. 22, 2014, which areall herein incorporated by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to the field of chemical operations and morespecifically to the field of decontamination of sour water by removinghydrogen sulfide.

Background of the Invention

There has been an increasing need to decontaminate storage tanks such aspetroleum refinery storage tanks that contain large volumes of sourwater. Such sour water typically contains hydrogen sulfide (H₂S).Conventional methods for treating sour water to destroy the H₂S includeusing an oxidizing agent, which typically converts H₂S to a variety ofnon hazardous sulfur compounds.

Conventional oxidizing agents include hydrogen peroxide, potassiumpermanganate, sodium persulfate, sodium hypochlorite,dimethyldodecylamine-N-oxide, and sodium perborate. Each of suchconventional methods has drawbacks. For instance, hydrogen peroxide istypically dangerous because the reaction between H₂O₂ and H₂S may bevery exothermic, with the heat of reaction potentially causing a violenteruption of boiling water. Drawbacks to potassium permanganate includethat the product of reaction with H₂S may be solid manganese dioxide,which is a solid and may add to sludge accumulation in the tank. Furtherdrawbacks to potassium permanganate include that the manganese dioxidemay accumulate in a tank in the presence of organics, which may lead tocombustion. Drawbacks to sodium persulfate include that the reactionwith H₂S may be very exothermic and also that its addition to asulfide-laden water may result in an exotherm, which may cause a rapidrise in temperature. Drawbacks to sodium hypochlorite include that itsuse may release toxic chlorine gas. Drawbacks todimethyldodecylamine-N-oxide include that its use may be impractical asit may be added in large quantities to sufficiently destroy the H₂S inlarge volumes of sour water. Drawbacks to sodium perborate include theamounts typically used to sufficiently destroy the H₂S may beimpractical.

Consequently, there is a need for an improved method for treating H₂S insour water.

BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS

These and other needs in the art are addressed in one embodiment by amethod for treating hydrogen sulfide in a solution. The method includesproviding the solution that contains hydrogen sulfide. The method alsoincludes adding sodium nitrite to the solution in an amount suitable toreact with the hydrogen sulfide and treat the hydrogen sulfide. In someembodiments, an ammonia-removal gas is used to remove produced ammonia.In other embodiments, the method also includes adding a pH buffer (i.e.,buffering agent), as needed, to maintain a neutral pH.

These and other needs in the art are addressed in another embodiment bya method for removing hydrogen sulfide from water, wherein the watercomprises hydrogen sulfide. The method includes adding sodium nitrite tothe water in an amount suitable to react with the hydrogen sulfide inthe water and remove hydrogen sulfide from the water. The sodium nitriteis added at a mole ratio to hydrogen sulfide in the water from about 1:3to about 2:3.

These and other needs in the art are addressed in a further embodimentby a method for removing hydrogen sulfide from water, wherein the watercomprises hydrogen sulfide. The method includes adding sodium nitrite tothe water in an amount suitable to react with the hydrogen sulfide inthe water and remove hydrogen sulfide from the water. The method alsoincludes blowing a gas onto the water to remove ammonia formed by areaction of sodium nitrite and the hydrogen sulfide.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other embodiments for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent embodiments do not departfrom the spirit and scope of the invention as set forth in the appendedclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an embodiment, sodium nitrite is added to a solution containinghydrogen sulfide and reacted with the hydrogen sulfide. The hydrogensulfide may be present in any concentration in the solution. Inembodiments, the hydrogen sulfide is present in high concentrations. Insome embodiments, a high concentration of H₂S is up to about 25,000mg/liter in the solution. In other embodiments, a high concentration ofH₂S is from about 2,000 mg/liter to about 25,000 mg/liter H₂S in thesolution, alternatively from about 5,000 mg/liter to about 25,000mg/liter of H₂S in the solution, and alternatively from about 10,000mg/liter to about 25,000 mg/liter in the solution.

The sodium nitrite may be added at any desired mole or weight ratio tohydrogen sulfide sufficient to treat the hydrogen sulfide in thesolution. To treat the hydrogen sulfide comprises destroying (i.e.,removing) the hydrogen sulfide in the solution. In an embodiment, thesodium nitrite is added at a mole ratio to H₂S from about 1:3 to about2:3 to treat the hydrogen sulfide. In some embodiments, the sodiumnitrite is added at a weight ratio to H₂S from about 0.676:1 to about5.0:1, alternatively from about 0.676:1 to about 1.353:1 to treat thehydrogen sulfide.

It has been discovered that the following Equations (1) and (2),respectively, occur when sodium nitrite is added to a solutioncontaining hydrogen sulfide to treat the hydrogen sulfide.NaNO₂+3H₂S→NH₃+3S⁰+NaOH+H₂O  (1)H₂O+4NaNO₂+6H₂S→2Na₂S₂O₃+(NH₄)₂S₂O₃+2NH₃  (2)

In an embodiment, a pH buffer is added to the solution. In someembodiments, the pH buffer is added to reduce the pH of the solution tobetween about 7.0 and about 9.0, alternatively to between about 7.0 andabout 8.5, further alternatively to between about 7.0 and about 8.0, andalternatively about 8.0. Without being limited by theory, the pH bufferis added in amounts to keep the pH level at or above 7.0 because sodiumnitrite may decompose to generate nitric oxides at pH values less than7.0. Further, without limitation, H₂S may be liberated in some instanceswhen large pH buffer amounts are added. The pH buffer may include anyacidic buffering chemical suitable for providing a stable pH in theneutral range. In an embodiment, the pH buffer includes citric acid,phosphoric acid, boric acid, or any combination thereof. In someembodiments, the pH buffer includes citric acid, phosphoric acid, or anycombination thereof. In alternative embodiments, the pH buffer isphosphoric acid.

It is to be understood that the reaction of sodium nitrite with thesulfide ion initially results in the destruction of sulfide with theproduction of elemental sulfur, thiosulfate, and ammonia, as shown byEquations (1) and (2). Without being limited by theory, the NH₃ and S⁰products enter a reverse reaction that reverts a portion of the productsulfur back through a polysulfide stage (S_(x) ⁵⁰) and then back to thesulfide ion (S^(═)). Further, without being limited by theory, suchreverse reaction may prevent or hinder the complete disappearance of thesulfide ion. Consequently, the pH buffer is added to reduce the pHlevels to a desired range, which removes the free ammonia and makes itpresent only as an ammonium salt.

The reactions of Equations (1) and (2) may proceed at any temperaturebetween about ambient temperature and about boiling temperature of thesolution. In an embodiment, the reactions occur between about 40° C. andabout 70° C., alternatively between about 40° C. and about 50° C.

In alternative embodiments, an ammonia-removal gas is blown onto thesolution. The ammonia-removal gas is blown to remove the ammonia. Theammonia-removal gas may be blown by any desired method. In anembodiment, the ammonia-removal gas is blown by a compressor. Forinstance, the solution may be disposed in a tank, and the sodium nitriteis added. The ammonia-removal gas may be blown into the tank to removethe ammonia. In some embodiments, the ammonia-removal gas is blown inaddition to the pH buffer addition. In alternative embodiments, theammonia-removal gas is blown in place of the pH buffer addition. Theammonia-removal gas may include any gas suitable for being blown andexposed to ammonia. In an embodiment, the ammonia-removal gas is airand/or nitrogen.

To further illustrate various illustrative embodiments of the presentinvention, the following examples are provided.

EXAMPLES

Direct potentiometric sensing of [S^(═)] at a Ag₂S electrode wasconducted. A calibration curve was prepared from a series of sulfidestandards at concentrations 10⁰, 10 ⁻¹, 10⁻², 10⁻³, and 10⁻⁴ M/L, witheach buffered 1:1 (by volume) with a sulfide anti-oxidant buffer. Datafor a standard curve of Emf vs. log [S^(═)] were taken daily, andstandard curves were calculated daily by linear regression analysis.

Two experiments were conducted on a sample of water that had beenanalyzed to contain 17,000 mg/L S^(═). Solid NaNO₂ was weighed into 50ml samples, and the tests were conducted in capped bottles with thesample and reaction products completely contained therein. Suchconditions were to approximate conditions within a tank during chemicaltreatment. All analysis samples were treated with a sulfide antioxidantbuffer (SAOB), which is a highly caustic buffer.

First Experiment

In one sample, the reaction was carried out at 40° C. The sample volumewas 50 mls. 0.353 mole/L (1.219 g/50 mis) NaNO₂ was added. S^(═) wasinitially present at 0.530 mole/L. The initial pH was 9.5. The resultsfrom the experiment are indicated in Table I.

TABLE I Reaction Time in Hours [S⁼], mg/L 0 16,541 2 4,422 4 716 664-358 8 96-199 12  6-139 24 61-105

In another sample, the reaction was carried out at 50° C. The samplevolume was 50 mis. 0.353 mole/L (1.219 g/50 mis) NaNO₂ was added. S^(═)was initially present at 0.530 mole/L. The initial pH was 9.5. Theresults from the experiment are indicated in Table II.

TABLE II Reaction Time in Hours [S⁼], mg/L 0 16,541 2 694 4 218 6 25-158 8  40-103 12 15-34 24 5-6

There was a strong presence of ammonia in the samples after onset of thereaction. The final pH of the samples was measured at a pH of 10.3. Toreduce the pH, a second experiment was conducted.

Second Experiment

The second experiment included periodic pH buffering and was carried outat 45° C. The sample volume was 50 mis. 0.260 mole/L (0.897 g/50 mls)NaNO₂ was added. S^(═) was initially present at 0.390 mole/L. Citricacid was added as a 50% concentrate with a specific gravity of 1.166.

Only the NaNO₂ was first added, and the initially fast reaction wasallowed to proceed unbuffered for two hours. After the two hour period,50% citric acid was added, which lowered the pH to about 8.1. The citricacid addition was repeated periodically during the reaction. The resultsare shown in Table III below. The two hour addition resulted insignificantly lowering the S^(═) in the sample from 3,200 mg/L to 180mg/L. It was seen that maintaining the pH at about 8.0 allowed theprocess of sulfide treatment (destruction) to proceed to completion.

TABLE III Reaction Time in Hours 50% Citric Acid Added [S⁼], mg/L 0 None12,501 1 None 7,349 2 3.0 mls (3.5 g)  3,197 3 1.5 mls (1.71 g) 178 50.42 mls (0.48 g)  18 6 None 2 7 None 0.04

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations may be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for removing hydrogen sulfide comprising: providing a solution comprising hydrogen sulfide; adding sodium nitrite to the solution comprising hydrogen sulfide; and adding a pH buffer to the solution to maintain a pH of the solution between about 7.0 to about 9.0, wherein the pH buffer removes free ammonia from the solution.
 2. The method of claim 1, wherein the concentration of hydrogen sulfide in the solution is about 2,000 mg/liter to about 25,000 mg/liter.
 3. The method of claim 1, wherein the sodium nitrite is added at a weight ratio to the hydrogen sulfide from about 0.676:1 to about 5:1.
 4. The method of claim 1, wherein the pH buffer comprises an acidic buffering chemical.
 5. The method of claim 1, wherein the solution further comprises water.
 6. The method of claim 1, wherein the adding of the sodium nitrite is performed at a temperature of from about ambient temperature to about a boiling temperature of the solution.
 7. The method of claim 1, wherein the adding of sodium nitrite is performed at a temperature of from about 40° C. to about 70° C.
 8. The method of claim 1, wherein a reaction between the hydrogen sulfide and the sodium nitrite produces ammonia.
 9. A method for removing hydrogen sulfide comprising: providing a solution comprising hydrogen sulfide, wherein the solution is disposed in a vessel; adding sodium nitrite to the solution comprising hydrogen sulfide; and adding a pH buffer to the solution to maintain a pH of the solution between about 7.0 to about 9.0, wherein the pH buffer removes free ammonia from the solution.
 10. The method of claim 9, wherein the concentration of hydrogen sulfide in the solution is about 2,000 mg/liter to about 25,000 mg/liter.
 11. The method of claim 9, wherein the sodium nitrite is added at a weight ratio to the hydrogen sulfide from about 0.676:1 to about 5:1.
 12. The method of claim 9, wherein the pH buffer comprises an acidic buffering chemical.
 13. The method of claim 9, wherein the solution further comprises water.
 14. The method of claim 9, wherein a reaction between the hydrogen sulfide and the sodium nitrite produces ammonia in the vessel.
 15. The method of claim 9, further comprising introducing a gas into the vessel.
 16. The method of claim 15, wherein the gas is nitrogen.
 17. The method of claim 15, wherein the gas is air.
 18. The method of claim 15, wherein the introducing of the gas into the vessel removes ammonia. 